Design of Task-Specific Ionic Liquids for Capturing CO2: A Molecular Orbital Study

To design task-specific ionic liquids (TSILs) for capturing CO2, the frontier molecular orbital interactions between CO2 and -NH2 in TSILs, such as 1,1,3,3-tetramethylguanidinium lactate and 1-n-propylamine-3-butylimidazolium tetrafluoroborate, were systematically studied by employing B3LYP/6-31G** calculations. A detailed frontier molecular orbital study showed that the inherent intramolecular hydrogen bonds in TSILs and the electron-donating groups attaching to -NH2 could raise the frontier occupied molecular orbital energy on -NH2 and thus enhance the interactions between -NH2 and CO2. Following the underlying interaction mechanism between CO2 and -NH2 in TSILs, two new TSILs, [P(C4)4][Ala] and [P(C4)4][Gly], were designed and the interactions between CO2 and -NH2 in such ionic liquids were investigated. This work showed a good prospect to design TSILs for selectively capturing CO2 from gas streams by applying the molecular orbital calculations.